SALISBURY, ENGLAND - MARCH 08: Forensic police officers wearing hazmat suits prepare to examine a vehicle believed to belong to Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in a critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

A picture shows the tent over the bench where a man and a woman were found in critical condition sparking a major incident on March 4 at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

Members of the emergency services in green biohazard encapsulated suits afix the tent over the bench where a man and a woman were found on March 4 in critical condition at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

A picture shows the tent over the bench where a man and a woman were found in critical condition sparking a major incident on March 4 at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

TOPSHOT - Members of the emergency services in green biohazard encapsulated suits work to afix the tent over the bench where a man and a woman were found on March 4 in critical condition at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

TOPSHOT - Members of the fire brigade in green biohazard encapsulated suits work to re-attach the tent over the bench where a man and a woman were found in critical condition sparking a major incident on March 4 at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

Members of the fire brigade get suited in green biohazard encapsulated suits to work to re-attach the tent over the bench where a man and a woman were found in critical condition sparking a major incident on March 4 at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

Members of the fire brigade are helped out of their green biohazard encapsulated suits by colleagues in white protective coveralls after an operation to re-attach the tent over the bench where a man and a woman were found in critical condition sparking a major incident on March 4 at The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 after the tent became detached.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Police officers search the home of Sergei Skripal in Salisbury who was found critically ill on a bench with his daughter on March 4 and were taken to hospital sparking a major incident on March 8, 2018 in Wiltshire, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Jack Taylor/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Police officers search the home of Sergei Skripal in Salisbury who was found critically ill on a bench with his daughter on March 4 and were taken to hospital sparking a major incident on March 8, 2018 in Wiltshire, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Jack Taylor/Getty Images)

A police officer guards a cordon on a residential street in Salisbury, southern England, on March 8, 2018 where police forensic tents are errected and police continue to work in connection with the major incident which started at The Maltings shopping centre in Salisbury on March 4.
British detectives on March 8 scrambled to find the source of the nerve agent used in the 'brazen and reckless' attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.
/ AFP PHOTO / Ben STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Forensic police officers wearing hazmat suits examine a vehicle believed to belong to Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Police stand guard at the home of Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in a critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Police stand guard at Zizzil restaurant reported to have been visited by Sergei Skripal before he colapsed due to poisoning on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in a critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Forensic police officers wearing hazmat suits examine a vehicle believed to belong to Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Forensic police officers wearing hazmat suits examine a vehicle believed to belong to Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Forensic police officers wearing hazmat suits prepare to examine a vehicle believed to belong to Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

SALISBURY, ENGLAND - MARCH 08: Forensic police officers wearing hazmat suits examine a vehicle believed to belong to Sergei Skripal on March 8, 2018 in Salisbury, England. Police investigations continue into the use of a nerve agent to poison Sergei Skripal, who was found ill in a Salisbury park with his daughter on March 4. Both Sergei Skripal and his daughter remain in critical condition in hospital. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

Pedestrians walk past a cordoned-off branch of the Italian chain restaurant Zizzi close to The Maltings shopping centre in Salisbury, southern England, on March 8, 2018 which was closed in connection to the ongoing major incident sparked after a man and a woman were found critically ill on a bench at the shopping centre on March 4 after being apparently poisoned with what was later identified as a nerve agent sparking a major incident.
Britain vowed on March 8 to act 'without hesitation' if a state is found responsible for a nerve agent attack on a former Russian double agent, as police said 21 people in total had received medical treatment following the incident. Sergei Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the attack on Sunday in the sleepy south-western English city of Salisbury.
/ AFP PHOTO / Adrian DENNIS (Photo credit should read ADRIAN DENNIS/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 09: Police officers search ahead of the visit by Home Secretary Amber Rudd to the scene at the Maltings shopping centre in where former Russian double agent Sergei Skripal and his daughter were found critically ill after exposure to a nerve agent on March 4 and were taken to hospital sparking a major incident, in Salisbury on March 9, 2018 in Wiltshire, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being exposed to an 'unknown substance'. (Photo by Matt Cardy/Getty Images)

SALISBURY, ENGLAND - MARCH 09: A police officer walks past a forensics tent over the grave of former Russian double agent Sergei Skripal's wife Lyudmila in London Road cemetery, on March 9, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being exposed to an 'unknown substance'. (Photo by Jack Taylor/Getty Images)

SALISBURY, ENGLAND - MARCH 09: A general view of a forensics tent over the grave of former Russian double agent Sergei Skripal's son Alexander in London Road cemetery, on March 9, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being exposed to an 'unknown substance'. (Photo by Jack Taylor/Getty Images)

SALISBURY, ENGLAND - MARCH 10: Police officers dress in protective suits and breathing apparatus in London Road cemetery as they continue investigations into the poisoning of Sergei Skripal who was found critically ill on a bench in Salisbury with his daughter sparking a major incident on March 10, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

SALISBURY, ENGLAND - MARCH 10: Police officers dress in protective suits and breathing apparatus in London Road cemetery as they continue investigations into the poisoning of Sergei Skripal who was found critically ill on a bench in Salisbury with his daughter sparking a major incident on March 10, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

British police officers stand on duty at a cordon leading to the home of Russian spy Sergei Skripal, in Salisbury, southern England, on March 10, 2018.
British soldiers were deployed on Friday to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 10: A police officer stands at a cordon at the scene connected to the Sergei Skripal nerve agent attack on March 10, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

Police tape surrounds a forensics tent covering the memorial stone to Alexander Skripal, inside the entrance of the London Road Cemetery in Salisbury, southern England, on March 10, 2018, where the wife and son of former Russian spy Sergei Skripal are buried.
British soldiers were deployed on Friday to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

TOPSHOT - A member of the emergency services wearing a protective suit works at the London Road Cemetery in Salisbury, southern England, on March 10, 2018, where the wife and son of former Russian spy Sergei Skripal are buried.
British soldiers were deployed on Friday to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Personnel in protective coveralls and breathing equiptment work at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Personnel in protective coveralls and breathing equiptment cover an ambulance with a tarpaulin at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel in protective coveralls stand by as personnel cover ambulances in tarpaulins at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Members of the emergency services in protective coveralls and breathing equiptment cover ambulances with tarpaulins at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Personnel in protective coveralls and breathing equiptment cover an ambulance with a tarpaulin at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel in protective coveralls stand by as personnel cover ambulances in tarpaulins at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Members of the emergency services in protective coveralls and breathing equiptment cover ambulances with tarpaulins and prepare to load them onto military low-loader vehicles at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Members of the emergency services in protective coveralls and breathing equiptment load a covered ambulance onto a military low-loader vehicle at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 10: Military personnel wearing protective suits inspect a bin after an ambulance is removed from Salisbury ambulance station as they continue investigations into the poisoning of Sergei Skripal on March 10, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

SALISBURY, ENGLAND - MARCH 10: Military personnel wearing protective suits cover two ambulances with tarpaulin as they prepare to remove them from Salisbury ambulance station as they continue investigations into the poisoning of Sergei Skripal on March 10, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

Personnel in protective coveralls and breathing equiptment cover an ambulance with a tarpaulin to be taken away by military personnel at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 10: Military personnel wearing protective suits cover a second ambulance after removing another from Salisbury ambulance station as they continue investigations into the poisoning of Sergei Skripal on March 10, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

Personnel in protective coveralls and breathing equiptment work at the Salisbury District Hospital in Salisbury, southern England, on March 10, 2018, in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack.
British soldiers were deployed on March 9 to help a counter-terrorism investigation into a nerve agent attack on former Russian spy Sergei Skripal, as speculation mounted over how London could retaliate if the Russian state is found to be responsible. Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the March 4 attack in the sleepy south-western English city of Salisbury. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 10: Soldiers wearing hazmat suits seal a waste container at an ambulance depot on March 10, 2018 in Salisbury, England. The ambulance is believed to have carried former Russian intelligence officer Sergei Skripal to hospital, after he and his daughter Yulia fell ill in Salisbury city centre on March 4. Both Sergei Skripal and his daughter remain in a critical condition due to the effects of an undisclosed nerve agent. The military have been called in to assist police in their investigation and containment of the major incident. Sergei Skripal was granted refuge in the UK following a spy swap between the US and Russia in 2010. (Photo by Rufus Cox/Getty Images)

Police officers stand in front of a cordon in front of a branch of the chain Italian restaurant Zizzi in Salisbury, southern England, on March 11, 2018, as investigations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

A pedestrian passes in front of a police cordon in front of a branch of the chain Italian restaurant Zizzi in Salisbury, southern England, on March 11, 2018, as investigations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

SALISBURY, ENGLAND - MARCH 11: Police officers stand outside Zizzi restaurant as it remains closed as investigations continue into the poisoning of Sergei Skripal on March 11, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

SALISBURY, ENGLAND - MARCH 11: A police officer stands outside The Mill as it remains closed as investigations continue into the poisoning of Sergei Skripal on March 11, 2018 in Salisbury, England. Sergei Skripal who was granted refuge in the UK following a 'spy swap' between the US and Russia in 2010 and his daughter remain critically ill after being attacked with a nerve agent. (Photo by Chris J Ratcliffe/Getty Images)

A police officer stands at a police cordon at The Maltings shopping centre in Salisbury, southern England, on March 11, 2018, as investigations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

A police officer stands in front of The Mill pub in Salisbury, southern England, on March 11, 2018, as investigations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
Traces of a nerve agent used in the suspected attempted murder of a Russian ex-spy have been found in a pub and a restaurant he visited, England's chief medical officer said on March 11. Sally Davies said up to 500 people who had visited The Mill pub and the Zizzi restaurant in Salisbury, southwest England, needed to wash their clothes and belongings as a precaution. The March 4 attack on Russian double agent Sergei Skripal and his daughter Yulia is being treated by detectives as attempted murder.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel, some in protective coveralls, carry equipment in a staging area at a cordon set up behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel, some in protective coveralls, talk at a staging area at a cordon set up behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

A military fork-lift vehicle is driven into an area cordoned off behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

A military low-loader vehicle is driven into an area cordoned off behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel wearing protective coveralls look at a map while others load a car onto a military vehicle in a cordoned off area behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel wearing protective coveralls look at a map while others load a car onto a military vehicle in a cordoned off area behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel wearing protective coveralls use a military fork-lift to lift a car onto a low-loader military vehicle in a cordoned off area behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel wearing protective coveralls use a military fork-lift to lift a car onto a low-loader military vehicle in a cordoned off area behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

Military personnel in protective coveralls handle equipment during an operation in a cordoned off area behind a police station in Salisbury, southern England, on March 11, 2018, as investigations and operations continue in connection with the major incident sparked after a man and a woman were apparently poisoned in a nerve agent attack a week ago.
British police hunting the perpetrators behind a nerve agent attack on a Russian former spy are examining more than 200 pieces of evidence, London said after a meeting of the UK's national emergencies committee. Counter-terror police had identified more than 240 witnesses, the Home Office interior ministry said after a meeting of the committee known as COBRA, chaired by Home Secretary Amber Rudd. Military personnel were assisting police for a third day after being called in to remove vehicles and items.
/ AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

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Britain is also set to freeze the accounts of individuals close to Russian president Vladimir Putin, as well as suspend all high-level contact with the Russian government.

The joint statement in full

We, the leaders of France, Germany, the United States and the United Kingdom, abhor the attack that took place against Sergei and Yulia Skripal in Salisbury, UK, on 4 March 2018. A British police officer who was also exposed in the attack remains seriously ill, and the lives of many innocent British citizens have been threatened. We express our sympathies to them all, and our admiration for the UK police and emergency services for their courageous response.

This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War. It is an assault on UK sovereignty and any such use by a State party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all.

The United Kingdom briefed thoroughly its allies that it was highly likely that Russia was responsible for the attack. We share the UK assessment that there is no plausible alternative explanation, and note that Russia's failure to address the legitimate request by the UK government further underlines its responsibility. We call on Russia to address all questions related to the attack in Salisbury. Russia should in particular provide full and complete disclosure of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons (OPCW).

Our concerns are also heightened against the background of a pattern of earlier irresponsible Russian behaviour. We call on Russia to live up to its responsibilities as a member of the UN Security Council to uphold international peace and security.

May today visited the site of the poisoning in Salisbury England to meet local people and those investigating the poisoning.